Polycation chain length determines the in vivo mRNA delivery efficiency of block copolymer micelles.

Messenger RNA (mRNA)-based therapeutics hold great promise by enabling in situ expression of therapeutic proteins. The success of these therapies in vivo critically depends on the design of safe and efficient delivery systems. Polymeric micelles formed by block catiomers can encapsulate mRNA, protect it from degradation, and facilitate intracellular delivery. Polycation chain length has been proposed as a design parameter of micelles, yet how it governs stabilizing interactions to define mRNA delivery performance remains unestablished. Here, we systematically investigate this relationship using a series of poly(ethylene glycol)-poly(glycidyl-phenylalanate) (PEG-PG(Phe)) block copolymers with varying PG(Phe) chain lengths. These polymers interact with mRNA via polyion complexation and π-π stacking. Our findings revealed that the block catiomers having PG(Phe) segments with 90 units formed micelles with superior stability and enhanced endosomal escape, while micelles with shorter segments were rapidly dissociated and micelles with longer blocks showed impaired PEG shielding. Moreover, the polymer with a PG(Phe) block of 90 units achieved 2-fold higher protein expression in vivo compared to shorter or longer chain variants. These findings reveal that tuning polycation chain length with core π-π interactions is key to unlocking highly efficient mRNA delivery.